Heavy breathing— an obscure link in asthma and obesity

There is a strong link
between obesity and asthma and as the prevalence of both conditions has been
increasing steadily, epidemiologists have speculated that there is an underlying
condition that connects the two. But one long-suspected link, the systemic
inflammation associated with obesity, has been ruled out by a recent
New
Zealand study that found no evidence of its
involvement.

“We were disappointed not to
find a ‘smoking gun’ that would explain the common association between obesity
and asthma,” said lead researcher, D. Robin Taylor, M.D., of the University of Otago in New Zealand. “However, this research
points us to other possibilities that future research should
examine.”

The results were reported in
the first issue for September of the American Journal of Respiratory and Critical
Care Medicine, published by the American Thoracic
Society.

In order to determine if
there was indeed an interaction between systemic and local inflammation, the
researchers recruited 79 women—20 who were obese with asthma, 19 who were of a
normal weight with asthma, 20 who were obese but who did not have asthma and 20
controls.

Asthmatics were told to stop
using their anti-inflammatory inhaler treatment to avoid confounding effects
until “loss of control.” After the withdrawal period of four weeks, subjects
underwent blood tests and tests for biomarkers of systemic and airway
inflammation, such as C-reactive protein (CRP) and cytokines in blood and
inflammatory cells and cytokines in sputum. Those that are known to be relevant
in both obesity and asthma were chosen. The researchers then analyzed for
interactions between systemic and airway-specific markers of
inflammation.

“What we found was that
although inflammatory cells and other biomarkers of inflammation were increased,
there was no significant interaction demonstrated between obesity and asthma,”
said Dr. Taylor.

Although their inflammation
hypothesis was not supported by their results, Dr. Taylor points out that it
does provide valuable direction for future research. “This does not change the
fact that there is a well-established link between asthma and obesity. Sometimes
a negative result is important, and the results add to our body of knowledge
regarding the obesity-asthma link. Now we need to look in other directions for
the answers.”

Animal studies suggest that
changes in innate immunity may occur with obesity. “We did not look at this in
our patients. Given that asthma is immunologically driven, this is a potential
avenue for further research,” said Dr. Taylor. “Alternatively, it may be that
dynamic changes in lung function that occur with episodes of asthma are
different with excess body weight.”

Whatever the link is,
uncovering it will have important clinical implications. “Obese patients with
asthma are more difficult to treat because their response to bronchoconstriction
is exaggerated and gives the impression that the asthma is worse,” said Dr.
Taylor. “They may well have worse symptoms, but not as a result of underlying
airway inflammation. Still, the typical response is often to increase their
inhaled anti-inflammatory therapy. This is unlikely to provide the answer and
may even do harm. The answer lies in dealing with the obesity itself.”